The present invention relates to a lightweight and high strength aluminum alloy having excellent resistance to heat and wear, particularly, to an aluminum alloy that can withstand use under extreme conditions. The invention also relates to a process for producing such an aluminum alloy.
Aluminum alloys are lightweight and resistant to corrosion. However, because of their low melting points, aluminum alloys have the inherent disadvantage of poor strength at elevated temperatures. Development efforts have been made to produce a heat- and wear-resistant aluminum alloy having a uniform structure of finely precipitated and crystallized grains by hot working a rapidly solidified aluminum alloy powder that permits alloy designs without limitation by the phase diagram. However, the technique of freezing a non-equilibrium phase by by rapid solidification presents problems in the subsequent and associated heating step in hot working. If the rapidly solidified alloy powder is heated for a certain period at a temperature suitable for hot working, the nonequilibrium phase converts to an equilibrium phase or the crystal grains grow to an unacceptably large size, thereby making it difficult to obtain a starting alloy that retains the microscopic features of the initial rapidly solidified powder. A material is necessary that can be softened during hot working but which exhibits an extremely high strength below that softening point.
With the recent demand for automotive engines and aircraft engines that perform better with less energy consumption, efforts are being made to reduce their size and weight while increasing the power output. In order to attain this object, materials used in pistons and other engine parts must be capable of withstanding very hostile conditions with respect to load and temperature.
Conventional pistons for automotive engines are cast from JIS AC8B and other Al-Si base alloys. However, alloys having a Si content of 20% or more have problems of segregation and coarsening of primary crystals (hypereutectics). It is not possible to produce castings adapted for service under high load and temperature conditions from such alloys having a Si content of 20% or more. In order to overcome these problems, considerable effort has been made to produce a high-temperature and wear-resistant aluminum alloy material which is pore-free and which contains uniform fine crystal grains by extruding or otherwise working a rapidly solidified high-Si aluminum alloy powder. However, the use of rapidly solidified powders requires careful selection of the fabrication method in order to avoid coarsening of grains due to hot-forming in the densification step. Furthermore, much technical difficulty is involved in adding dispersion particles to the rapidly solidified powder. In other words, heretofore, there has been no success in providing an advanced high-temperature and wear-resistant aluminum alloy simply by means of dispersion strengthening based on rapid solidification techniques.